Method and system for detecting and locating single-phase ground fault on low current grounded power-distribution network

ABSTRACT

A method and system for detecting and locating a single-phase ground fault on a low current grounded power-distribution network, comprising: respectively testing and picking up the voltage signals and current signals at multiple positions on each phase feeder ( 61 ), and determining the corresponding transient voltage signals and transient current signals according to the extraction of the voltage signals and the current signals ( 62 ); when the change in the transient voltage signals and the transient current signals exceeds a preset threshold ( 63 ), synchronously picking up the voltage signals and current signals at multiple positions on a three-phase feeder ( 64 ); calculating corresponding zero-sequence voltages and zero-sequence currents according to the voltage signals and current signals synchronously picked up at multiple positions on the three-phase feeder ( 65 ), and then extracting the steady-state signal and transient signal of the zero-sequence voltage and zero-sequence current at each position on the three-phase feeder ( 66 ); and determining a specific fault location on a faulty line according to the steady-state signal and the transient signal ( 67 ). The method effectively detects and displays a single-phase ground fault on a low current grounded power-distribution network.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2014/074206, filed in the Chinese Patent Office on Mar. 27, 2014,which is a continuation of Chinese Application No. 201310106380.2, filedon Mar. 29, 2013, and 201310120519.9, filed on Apr. 9, 2013, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to the field of electronic technology,particularly to a low current single-phase ground fault detection andlocation methods and systems.

Power system contains power supply from power plants. Power istransferred to the load side first by high or extra high voltagetransmission network, and then by the lower voltage levels ofdistribution network.

A common fault in the main power grid is short circuit and ground fault.Short circuit faults include three-phase and two-phase short circuitfaults. A common ground fault type is a single-phase ground. Shortcircuit fault detection technology has been very mature. But forsingle-phase ground fault detection, especially for low current groundeddistribution network single-phase ground, there is no effective method,and is recognized as a worldwide problem.

China and some countries are using mostly low current grounded griddistribution network. Therefore, the vast majority of failures aresingle-phase ground. The main advantage of low current groundeddistribution grid is that: in a single-phase ground fault, unlike ashort circuit, the system produces only a low ground current, therefore,the three-phase line voltage is still symmetrical, and does not affectthe normal operation of the system. China regulations require if asingle-phase ground fault occurs, the low current grounded distributionnetwork should continue to operate with fault 1˜2 h. Because of thisreliability, the low current grounded distribution network has beenwidely used.

However, single-phase ground fault must be found as quickly as possibleduring single-phase ground fault troubleshooting. Otherwise, the overvoltage caused by the failure of the ground fault can cause the cable toexplode, the voltage transformer PT to burn down, the bus to burn andother power system accidents. If not fixed in time, long-running groundfault will burden local residents, livestock with tremendous securityrisks.

The grounded mode of the low current grounded power distribution networkis either not grounded or grounded by the arc suppression coil.According to DL/T 1997-620 “AC electrical equipment over voltageprotection and insulation coordination”, in the 10 kV distributionnetwork, which is composed of pure overhead line or overhead line andcable, if the capacitor current is less than 10 A, it can be usedungrounded. But when the capacitor current is greater than 10 A, it isnecessary to install the arc suppression coil.

Distribution network with either neutral point ungrounded or arcsuppression coil grounded mode, when the single-phase ground occurs in,does not produce a large fault current in A, B, C phases, the followingdetails the two cases:

1 Distribution Network with Neutral Point Ungrounded Method

When the distribution network with neutral point N is not grounded, asshown in FIG. 1, in one of the three phases A, B, C, such as A phase,assume a ground fault occurs. In the initial phase failure, phase Avoltage will drop rapidly, voltages of non-fault phase B and phase Cwill rise rapidly, the neutral point voltage also will rise rapidly.While the A-phase feeder ground distributed capacitance will take placequickly discharging to ground, B phase and C phase feeder grounddistributed capacitance rapidly charged through the ground to form ashort-lived charge-discharge process (10 ms˜20 ms), have a relativelylarge transient capacitive current. Than the system will enter intosteady state power by way of the non-fault phase distributed capacitanceand ground, resulting in a continuous steady state capacitive current.In this process, the transient capacitive current is much larger thanthe steady-state capacitive current.

Adding all the three-phase line currents, one can get the linezero-sequence current. Similarly, adding the three-phase line voltagestogether, one can get the line zero-sequence voltage. In this process,the fault line zero sequence current is shown in FIG. 2. Before a groundfault occurs, the line zero-sequence current is very low, approximatelyzero. When a ground fault occurs, first there is the capacitor chargeand discharge transient process, resulting in a relatively large highfrequency transient capacitive current, and then to maintain a majorenergy concentrated in the frequency (50 Hz or 60 Hz) of the steadystate capacitive current.

2 Distribution Network Neutral Point Petersen Coil Grounded Method

When the distribution network with neutral point N grounded through thePetersen coil, shown in FIG. 3, in one of the A, B, C three phases, forexample, phase A, assume a ground fault occurs. Similar to theungrounded method, in the initial part of the failure, phase A voltagewill drop rapidly, voltages of non-fault phase B and phase C will riserapidly. Also will rise is the neutral voltage. Next, the A-phase feederground distributed capacitance quickly discharged to ground throughgrounded points. B phase and C phase feeder ground distributedcapacitance rapidly charged through the ground to form a short-livedcharge-discharge process (10 ms˜20 ms), forming a relatively largetransient capacitive current. Next, the suppression coil L will producea compensating current to compensate for the non-fault phase powerthrough the distributed capacitance, and in the process, forming groundcapacitive current comparable to a steady state. Last, the system entersthe steady state. Using neutral point arc suppression coil groundedmethod, the steady capacitance fault line current will become very low,and unlike FIG. 2, no large steady-state zero-sequence current isproduced. But in the fault line, the transient capacitive current is notaffected.

In low current grounded method, in particular by Petersen coil, in thedistribution network single-phase ground fault situation, theinstantaneous fault current duration is very short, and the steady-statefault current is very low. Data shows that the vast majority ofsingle-phase ground fault grounded resistance is greater than 800Ω,belongs to the high-impedance ground fault, such as through thebranches, the grass, the damp earth walls, etc., such that theinstantaneous fault current is not large, with very short duration of10˜20 ms. Thus in a low current grounded method with a single-phaseground fault, the detection and location is recognized as a worldwideproblem. There are several methods and apparatus for low currentsingle-phase ground fault detection:

1 Substation Low Current Grounded Line Selection Means

Existing low current grounded line selection device allows thesubstation bus to identify which line ground fault has occurred. InFIGS. 1 and 3, for example, there are two lines—a fault line and anon-fault line. Through a low current grounded line selection device,one can identify the fault line.

A low current grounded line selection device works by monitoring zerosequence current residual voltage at substation bus and each branchline. By treating a sudden increase in the zero-sequence voltage in thegrounded line as a trigger condition, follows by using the variousbranches of the line the zero-sequence current steady state informationand transient information, the device will identify the fault line.According to the use of different information, the device operation canbe classified as of steady line mode or transient line mode.

Steady line determination is mainly based on:

(1) The zero sequence current amplitude is maximum at the fault line;

(2) The fault line's zero-sequence current phase is opposite of that ofthe non-fault line;

(3) The fault line's zero-sequence reactive power is negative;

(4) The fault line's zero-sequence active power is large;

(5) The current of 5th harmonic is large at the fault line and isopposite of that of the non-fault line;

(6) The fault line's negative sequence current is large.

Transient line determination is mainly based on:

(1) The non-fault line and the fault line differs in that when phasevoltage reaches zero-sequence, the transient current and voltage of thefirst half-wave amplitude and phase are different;

(2) The use of other processing methods such as from the zero-sequencecurrent characteristics of transient information, extracting a low wave,and from there, with artificial intelligence, such as neural networks,identifying the fault line and non-fault line.

The main disadvantages of the low current grounded line selection deviceare:

-   -   (1) The existing substation PT (voltage transformer) and CT        (current transformer) affect the reliability and accuracy of the        selected line.

For the low current grounded line selection device to trigger, the buszero-sequence voltage signal must be connected in parallel on the bus PTto be obtained. Selection device is subject to PT ferromagneticresonance, and it will cause significant interference.

Because factors such as the special zero-sequence CT volume, high cost,and the need to install power, for low current grounded line selectiondevice to obtain zero sequence current is not usually obtained throughspecial zero-sequence CT, but by a substation having three-phase ortwo-phase measurements obtained with CT. The ideal CT, is one with nomagnetizing current consumption, the ampere-turns of the primary and thesecondary coils are equal in value, the primary current and thesecondary current phase measurements are the same and there is no phaseshift. In a practical CT, there is an excitation current, thus theampere-turns of the primary and the secondary coils are not equal, andthe primary current phase and the secondary current phase are not thesame. Therefore, due to the actual CT usually having angular error andphase change error, resulting in an unbalanced three-phase CT. Also thethree-phase CT superposition of zero-sequence current is an unbalancedcurrent, and the actual zero-sequence current errors exist, that wouldimpact of the line election results. In addition, in traditionalmeasurement of CT, due to the magnetic core having non-linear excitationcharacteristics, there is an impact on the current linearity from lowcurrent to large current. When the current is large, the core exhibitsmagnetic saturation, which will lead to CT saturation. In practice, themid to low voltage grid frequently exhibits the CT saturationphenomenon, causing failure in getting the correct zero sequence currentin performing line selection. Moreover, the core maintains energystorage cycle and magnetizing cycle, which makes CT transientcharacteristics not satisfactory. When current changes are not properlyfollowed, it is difficult to accurately capture weak transient signals.

-   -   (2) The location of the ground fault cannot be accurately        located.

Low current grounded line can only be installed in power distributionbus to sub location, and only for selecting a ground fault in the branchline. It is not capable to locate the position of branch line where theground fault occurs.

2 Signal Injection Method and Apparatus

Signal injection method through the injection signal source with faultdetection and location indicator may be used for detecting a largeground current on a permanent ground fault. The principle of this methodis: when the substation detects that the zero-sequence voltage increasessignificantly and continue for some time, together with thezero-sequence current being greater than a threshold value, and continuefor some time, it can be determined that the ground fault occurs; uponthis determination, Petersen coil is required and inserted; thereafter,by inserting at the neutral point of the transformer with a certainpattern of fault current signal; because the fault current signal can bedetected by the fault indicator at a location before the ground fault,but not at a location after the ground fault, the location of the groundfault can be traced.

As seen in FIG. 4, the ground source is connected between the neutral ofthe grounded substation transformer and ground with controlled resistiveloads (mid-range resistance, typically 100Ω). When ground fault isdetected, using micro controller, at the neutral of the groundedsubstation transformer (in the case of non-grounded transformer, the busneutral point), the resistive load source is automatically deployed fora short period of time, so that between the substation and field grounda special coded low signal current is produced. By resistive loadswitching with ground source code control, it can generate asuperimposed load current embedded with coding in the current signal. Byinstalling the ground fault indicators though out at the line and at thebranch point of the substation, the detection of the current signalcould be automated, and therefore, the goal of locating the fault isachieved.

Injection signal source method has the following disadvantages:

(1) It requires the installation of the signal source in the substation,therefore, changing the system operation;

(2) The signal sources and other devices require additional investmentand construction, and the construction process requires a power outage;

(3) For ground fault at the common resistance of 800Ω or more, thismethod was unable to produce a sufficient large coded signal current tobe detectable by fault indicator, therefore, this method cannot detecthigh-resistance ground fault;

(4) It cannot detect transient ground faults.

3 Network Feeder Terminal Unit FTU Based Method for Ground FaultDetection and Location

FTU network based ground fault detection and location method is shown inFIG. 5. The method works by installing switches and related detectionterminal FTU throughout the line and wiring them in a network, so torecord three-phase current, voltage waveform data during ground faultand send the data to the automated central system to be analyzed inorder to determine within which switch the fault lies.

The main disadvantages of the FTU network based ground fault detectionand location method is:

(1) There is a need to install the switches, and the switches must havean internal CT and PT, with switches and FTU being a huge investment;

(2) In measuring CT, it would be difficult to capture the transientsignals; also it would be facing high current saturation; and during thethree-phase superimposed zero-sequence, phase imbalance is causing largeerrors; these are factors making high resistance grounded difficult tobe detected;

(3) PT has ferromagnetic resonance problem;

(4) Installing switch and FTU needs power lines to be offline;

(5) It can only locate the fault up to between switches, but not more interms of accurate positioning;

(6) For overhead line to provide power to the FTU is very difficult,negatively affecting its installation and functioning.

At present time, the low current grounded system, especially the neutralpoint Petersen coil grounded system, the lack of effective ground faultdetection methods and equipment makes it hard to detect the ground faultposition. Many electricity departments are still using manual diagnosticmethods such as cable method to locate the ground faults. These methodsare of low degree of automation, are difficult and inefficient toimplement, and are unable to meet the requirements of the power systemto continue to improve the reliability of power supply. These methodsare also making it difficult to improve power quality and power supplyreliability. In order to improve the distribution network for lowcurrent grounded power supply reliability, it is necessary to provide amethod and apparatus, when the single-phase distribution network hasaground fault, whether it is based on low resistance or high resistancegrounded, or whether it is instantaneous fault or a permanent fault, toeffectively detect and locate the fault.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a low currentsingle-phase ground fault detection and location, which can determinethe location timely and accurately.

The purpose of the present invention is achieved by the followingtechnical solutions embodied in a method, a device and a system:

A low current single-phase ground fault detection and location method,comprising:

-   -   monitoring and extracting on each phase feeder the voltage and        current signals in multiple locations, and to determine the        corresponding transient voltage signal and transient current        signal based on the voltage and current signals;    -   When the degree of change in the transient voltage signal and        the transient current signal exceeds a set threshold,        synchronously extract the three-phase feeder voltage and current        signals at a plurality of positions;    -   based on the synchronously extracted three-phase voltage and        current signals at the plurality of positions, calculate the        corresponding zero-sequence voltage and zero-sequence current;    -   based on the steady state vs. transient zero-sequence voltage        and zero-sequence current values, determine the fault line, and        then further determine the specific location of the fault line.    -   The transient voltage signal and the transient current signal        level change data includes: the individually calculated        magnitude of each of the transient voltage and current signals,        the average, the differential value and the integral value of        one or more of the degrees of change.    -   The steps of synchronously extracting the voltage and current        signals at the plurality of positions on the three-phase feeders        include: using time division multiplexing wireless communication        network timing and GPS timing to synchronize time and then based        on the synchronized time to extract voltage and current signals        on the three-phase feeders at multiple locations.    -   The steps to determine the fault line based on the zero-sequence        voltage and zero-sequence current in steady state include:        extracting the steady state characteristic values at various        locations for the zero sequence voltage and zero sequence        current, the characteristic values include: one or more of        amplitude, average, differential value and integral value; also        extracted are steady state zero sequence active power and        reactive power sequence; extracting the steady-state waveform of        the zero-sequence voltage and zero-sequence current signals at        various locations; by comparing after-fault data with the steady        state data of each phase line and the waveform similarity and        difference to determine the fault line;    -   The steps to determine the specific location of the fault line        based on the zero-sequence voltage and zero-sequence current        transient signals include: extracting the transient        zero-sequence voltage signal and zero sequence current for each        position of the line before the determined fault; extracting a        set of characteristic values of the transient signals, the        characteristic values include: one or more of amplitude,        average, differential value and integral values, as well as at        the various locations the transient zero sequence active power        and reactive power, also calculated are transient zero-sequence        voltage and zero-sequence current signal waveform similarity at        various locations; and based on comparing the fault line signal        waveform similarity and the characteristic values, by comparing        the differences of the two at the different locations, to derive        at the ground fault location of failure on the line.    -   The method further comprises: in determining the ground fault        location, the geographic information system (GIS) map is used to        show the ground fault point, and to display the ground fault        information by sending over a wireless communication network the        fault indication unit at the ground fault point.

A feeder monitoring device, comprising:

-   -   feeder parameter monitoring module for picking up the detected        voltage and current signals in each phase feeder at multiple        locations, and based on the voltage and current signals to        determine the corresponding transient voltage and transient        current signals; when the degree of change of the transient        voltage signal and the transient current signal exceeds a set        threshold, send a notification to other adjacent phase feeder        monitoring devices to synchronize signal pickup in the other        monitoring modules;    -   signal synchronization pickup module for receiving the notice        sent by the adjacent feeder monitoring device's feeder        parameters monitoring module, performing synchronization and        picking up the voltage and current signals synchronously with        the other device's signal synchronization pickup module, and        reporting to the sending device.    -   The said signal synchronization module would include a        synchronization processing module. The synchronization        processing module is synchronized by time division multiplexing        using the wireless communication network's timing coupled with        the GPS timing, thereby enabling the said signal synchronization        pickup module to be based on time synchronization regarding the        voltage and current signals pickup at each phase on the feeder.    -   The apparatus further comprises: fault indication module, for        after determining a ground fault to indicate the location of the        point of failure, particularly implemented with wide angle        bright LED lights, and through a combination of different        numbers of diodes, as well as though flashing intervals and        frequency to indicate the different types of failure.    -   The said feeder parameter monitoring module uses capacitive        voltage sensors to pick up the voltage signal, and uses        electronic current sensors to pick up the current signal. The        electronic current sensor includes: current transformers, wound        or printed circuit Rogowski coil. And when the feeder parameter        monitoring module detects that the transient voltage signal and        the transient current signal exceed a set threshold, notify        through infrared, sound, ultrasound or magnetic signal as a        trigger to signal to the adjacent feeder monitoring device, with        its signal synchronization pickup module. The notification        method comprising:    -   directly transmitting notification to the adjacent feeder        monitoring devices on the other two phases, for their signal        synchronization pickup module;    -   or,    -   transmit notification to a second adjacent monitoring device in        a second phase and then let the second monitoring device to        transmit to a third adjacent monitoring device on a third phase,        for their signal synchronization module;    -   or,    -   transmit notification to a communication terminal device within        the phase, then let the communication terminal device forward        the trigger signal to the other two monitoring device in the        other two phases.    -   The apparatus further comprises:    -   a wireless communication module, for communicating with the        other two feeders monitoring device so that a wireless        communication is established by time division multiplexing        manner with the other two feeder monitoring devices in a        corresponding position, and for collecting from the other two        feeder monitoring devices the parameters, and for forwarding        them to a system master station through the wireless        communication network.    -   The apparatus further comprises:    -   a power requisition module, which has the ability to obtain        electrical power from a closed magnetic circuit by a latching        mechanism so to supply power to the feeder monitoring device;    -   a power management module, for controlling the power requisition        module, and for providing battery-backup power supply;    -   a power level control module, for leveling the power consumption        by the power requisition module.    -   The apparatus further comprises a feeder clamping member using        springs, a line support and a locking means, for clamping to the        feeder line between the line support and the springs. The line        support can be adjusted for the feeder position. The said feeder        clamping member can hold the feeder in tight closure after        clamping to the feeder by the locking means.

A low current single-phase ground fault detection and location system,including the said feeder monitoring device and a fault location unit,having:

-   -   the fault location unit, for based on the synchronously        collected voltage and current signals, to calculate for the        three-phase feeder sat plurality of positions, the corresponding        zero-sequence voltage and zero-sequence current, as well as to        extract at the various locations on three-phase feeder steady        state and transient signals from the zero-sequence voltage and        zero-sequence current; based on the steady signal of the        zero-sequence voltage and zero-sequence current to determine the        line for which a ground fault has occurred; and based on the        transient signal on the zero-sequence voltage and zero-sequence        current, to determined that the failure is at a specific fault        location on the line.    -   The fault location unit comprises:    -   a fault line location module, for extracting stationary signals        at various locations, including the zero sequence voltage, zero        sequence current, and characteristic values of the steady-state        signal; with the characteristic values include: amplitude,        average, differential value, integral value or one or more of        those; and calculate at the various positions steady zero        sequence active power, zero-sequence reactive power; further        calculating the steady-state waveform similarity of the        zero-sequence voltage and zero-sequence current signals at        various locations; and based on differences in the steady-state        signal characteristic values and the waveform similarity at each        phase line, to determine the selection of the fault line;    -   a fault point positioning module, for extracting at the        respective positions at the fault line indicated by the fault        line location module, the zero-sequence voltage and        zero-sequence current transient signals; calculating the        transient signal characteristic values, with the characteristic        values include: amplitude average, differential value, integral        value or one or more of those; also calculating at the various        positions, the transient zero sequence active power and reactive        power zero-sequence; further calculating the waveform similarity        of the transient zero sequence voltage and zero-sequence current        signals; and determining the ground fault point position based        on differences in the transient waveform similarity and the        characteristic values at each position.    -   The system also includes:    -   a fault display and indication processing unit for after        determining the ground fault point, showing it on the geographic        information system (GIS) map, and sending a ground fault signal        in a wireless communication network to a fault indication unit        near the ground fault point for display.    -   The system may also include:    -   a communication terminal device for communicating with the        feeder monitoring device to relay the received parameters up to        the system central unit's fault locating unit.    -   For each of said communication terminal device and the one or        more sets of three of the three-phase feeder monitoring devices,        the time division multiplexing method is used to establish        wireless communication between a communication terminal device        with the feeder monitoring devices. The said communication        terminal device and the said system central unit communicate via        a wireless communication network.    -   The system may also include:    -   a communications terminal equipment, instead of communicating        with the set of three-phase feeder monitoring devices above,        only communicates directly with any one of the three-phase        feeder devices, and transmit all three feeder monitoring        devices' parameters up to the system's central fault locating        unit; for the collection of parameters from the other two feeder        monitoring devices, wireless communication is established        between the first feeder monitoring and rest of the two feeder        monitoring devices, with time division multiplexing method used;        the said communication terminal device and the said system        central unit communicate via a wireless communication network.    -   The system may also include:    -   a communication terminal embedded feeder monitoring devices that        have built-in communication terminal functions; with the        communication terminal embedded feeder monitoring devices used        to communicate with the other two-phase feeders in the        corresponding set; the set uses time-division multiplexing        wireless communication to send parameters to the enhanced feeder        monitoring device; the communication terminal embedded feeder        monitoring devices then forwards the parameters to the system        central unit's fault locating unit via a wireless communication        network.

From the said technical solution provided above by the presentinvention, one can understand with the embodiments of the presentinvention, after a low current single-phase ground fault has occurred,the invention can be used to detect and locate the ground fault,regardless of whether it is low resistance or high resistance grounded,or whether it is a transient ground fault or permanent ground fault, allwith effective detection and indication.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solution of theembodiments of the present invention, the following figures areprovided. Obviously, the figures are only describing some embodiments ofthe invention. For one of ordinary skill in the art, other figures canalso be obtained based on these figures.

FIG. 1 shows at the neutral point N, the ungrounded single-phase groundfault currents.

FIG. 2 shows after a single-phase ground occurs, the fault line zerosequence current.

FIG. 3 shows the neutral point N, the Petersen coil groundedsingle-phase ground fault current.

FIG. 4 shows how the injection signal source ground fault detectionmethod works.

FIG. 5 shows the principle of the networking FTU ground fault detectionmethod.

FIG. 6 shows a flow chart of the method invention embodiment.

FIG. 7 provides a schematic diagram of the apparatus inventionembodiment.

FIG. 8 shows the apparatus invention embodiment in the open state beforebeing clamped to the feeder by clamping mechanism.

FIG. 9 shows the apparatus invention embodiment in the closed stateafter being clamped to the feeder by clamping mechanism.

FIG. 10 shows a schematic diagram of the apparatus which implements manyfeatures.

FIG. 11 shows a schematic diagram of apparatus implementing the faultindication unit.

FIG. 12 shows a schematic diagram of the system invention embodiment.

FIG. 13A shows how a single-phase ground fault will trigger the feedermonitoring unit to monitor the transient voltage, and how the currentdramatically changes.

FIG. 13B shows after the feeder monitoring unit detects the suspectedground fault, it synchronously triggers other similar feeder monitoringunits via wireless.

FIG. 13C shows using wireless communication transmission, the voltageand current waveform data of three-phase feeder monitoring unit can besent to the communication terminal, and then be uploaded to the systemcentral unit.

FIG. 13D shows the system central unit's platform software, after makingthe ground fault detection, issues a ground fault signal to thedistribution network feeder monitoring unit positioned before the faultto indicate the fault location.

FIG. 14A shows how phase A will trigger both phase B and phase Csynchronously over wireless.

FIG. 14B shows how phase A will trigger phase B, and then phase B willtrigger phase C synchronously over wireless.

FIG. 14C shows how the phase A will trigger the communication terminalfirst, before the communication terminal will trigger phase B and phaseC synchronously over wireless.

FIGS. 15A, 15B, 15C, and 15D show the four embodiments of the systeminvention.

DETAILED DESCRIPTION OF THE INVENTION

Below in connection with the accompanying figures of the presentinvention embodiment, the present invention will be apparent in thetechnical implementation of the invention. The described embodiments areonly part of the present invention, but not all. The describedembodiments of the present invention, and all other embodimentsperceived by those of ordinary skill in creativity, all belong to thescope of the present invention.

Below with references to the figures, the present invention is furtherdescribed in detail.

An embodiment of the present invention is to provide a low currentsingle-phase ground fault detection and location, as shown in FIG. 6,comprising:

At step 61, data collections were made to voltage and current signalsfor each of a plurality of positions in the feeder;

Specifically at set intervals, periodically the voltage and currentsignals on each phase feeders are collected at the plurality ofpositions, or they can be collected at specified times. Alternatively,one can use other ways to set the schedule on each phase feeder tocollect voltage and current signals at the plurality of positions.

At step 62, the voltage and current signals are picked up according tostep 61 to determine the corresponding transient voltage signals andtransient current signals.

Step 63 is to determine the extent of change in the transient voltagesignals and the transient current signals. If they exceed a setthreshold, then the method proceeds to step 64. Otherwise, it returns tostep 61.

Among these steps, the degree of change in the said transient voltagesignals and transient current signals comprises:

The transient voltage and current signals from each calculation are usedto determine the magnitude, the average value, the differential valueand integral value, as the one or more degrees of change. For example,one can calculate the results and compared them between two adjacentpositions to determine the extent of the change, or one can calculatethe present results and compare with the previous average of multiplecalculation results to determine the degree of change, or one can alsouse other measures in a predetermined manner to determine the degree ofchange.

Specifically, one can pick up the voltage and current signals withband-pass to get the transient voltage signal and the transient currentsignal, then extract from the transient voltage signal and transientcurrent signal after band-pass treatment the amplitude, the average, thedifferential value, the integration value, or any one or more thereof.When one or more of the above values change to exceed a set threshold,with step 64, the suspected fault triggers the ground fault alarm. Atstep 64, after the degree of change in the transient voltage signal andthe transient current signal exceeds a set threshold, the synchronizedcollection of voltage and current signals at a plurality of positions istriggered at the three-phase feeders.

The mode for the three-phase synchronous collection of feeder voltageand current signals of the plurality of positions may include:

time division multiplexing wireless communication network timing and GPStiming with synchronization; the synchronized pickup of voltage andcurrent signals at the three-phase feeder at the plurality of positionsis based on the synchronization of time.

After generating a suspected ground fault alarm, specifically as atrigger by infrared, sound, ultrasound, magnetic or electromagneticfield signals, one feeder will trigger the other two-phase feeders.Specifically, the trigger signal can be directly transmitted to theother two-phase feeders. Alternatively, the first trigger signal istransmitted to an adjacent phase feeder, and then transmitted by theadjacent phase feeder to the other phase feeder. Another alternative isthe trigger signal is first sent to the neighboring wirelesscommunication terminal device, and then transmitted via thecommunication terminal device to other two-phase feeders. Upon receivingthe triggers, the other two-phase feeders will upload the voltage andcurrent waveforms signals to be used for calculating the zero-sequencevoltage and zero-sequence current signals for detecting ground faults.

At step 65, based on the voltage and current signals picked up atthree-phase synchronized feeders at a plurality of positions, thecorresponding zero-sequence voltage and zero-sequence current arecalculated.

At step 66, the extraction of steady state signals and transient signalsis performed on the three-phase feeders at the respective positions, forthe zero sequence voltage and zero sequence current.

At step 67, based on the steady state signal the zero-sequence voltageand zero-sequence current, determination is made as to the fault line.And further based on the zero-sequence voltage and zero-sequence currenttransient signals, determination of the specific position of the faulton the faulty line is made.

At the step 67, the process of using the steady state signals of thezero-sequence voltage and zero-sequence current to determine the faultline may include:

extracting from the steady signals at a plurality of locations the zerosequence voltage and zero sequence current, the characteristic values ofthe steady state signals, the characteristic values include: amplitude,average, differential value and integral value, or one or more thereof;also calculated are the zero sequence active power and reactive powersequence; further calculated are the similarities from steady-statewaveform zero-sequence voltage and zero-sequence current signals at theplurality of locations; determination of fault line based on thedifference of steady state signal characteristic values and the waveformsimilarities from the different phase lines.

At the step 67, based on the zero-sequence voltage and zero-sequencecurrent transient signals, the method to determine the specific positionof the fault on the faulty line may include:

extracting from the transient signals at the plurality of locations thezero sequence voltage and zero sequence current, the characteristicvalues of the transient state signals, the characteristic valuesinclude: amplitude, average, differential value, integral value, or oneor more thereof; also calculated are the zero sequence active power andreactive power sequence; further calculated are the similarities fromtransient-state waveform zero-sequence voltage and zero-sequence currentsignals at the plurality of locations; determination of fault lineposition based on the differences of the transient state signalcharacteristic values and the waveform similarities at the plurality oflocations.

After performing the above step 67 to determine the ground fault point,the result can be displayed on the map GIS (geographic informationsystem) by sending over a wireless communication network the groundfault information to a fault indication unit for ground fault pointdisplay, for inspectors to quickly and easily find the location of thefault.

Embodiments of the present invention further provide a feeder monitoringmeans for installation at a plurality of locations on each phase of thefeeders. The specific structure is shown in FIG. 7, which may include:

a feeder parameter monitoring module 71 for monitoring and collecting ofthe voltage and current signals on each phase feeders at a plurality oflocations, and based on the data for calculating the correspondingtransient voltage signal and the transient current signal, and based onnoticing the degree of change from the transient voltage signal andtransient current signal exceeds a set threshold, for notifying thefeeder monitoring device from the adjacent phase feeder to perform thesignal synchronization module 72.

Signal synchronization pickup module 72 is for receiving notice sent bythe feeder parameter monitoring module on the adjacent monitoring deviceto facilitate synchronized collection of voltage and current signals onthe feeder. Doing so is for achieving the synchronized collection ofvoltage and current signals from the three-phase feeders, and forreporting the data to the system central unit. The signalsynchronization module 72 specifically includes a synchronizationprocessing module 721. The synchronization process module 721 performstiming synchronization by time division multiplexing wirelesscommunication network timing and GPS timing, and thus ensures the signalsynchronization pickup module 72 can be based on the synchronized timefor collection from the corresponding position all three-phase feedersthe voltage and current signals.

Further, the apparatus may further include a fault indication module 73for after determining there is a ground fault, to indicate where thefault is located. For example, the fault indication module can light upa corresponding indicator for inspectors to notice the fault position.As for the indicator, wide angle bright LED lights can be specificallyused in all directions to ensure the maximum attraction in 360 degreecoverage, and can be further configured with different numbers ofdiodes, flashing sequence and frequency to indicate the different typesof fault.

In the feeder monitoring devices, the corresponding parameter monitoringmodule 71 can specifically use capacitive voltage sensors to pick up thevoltage signal, and use electronic current sensors to pick up thecurrent signal. The electronic current sensors include: currenttransformer, wound Rogowski coil or printed circuit Rogowski coil, andthe like. When the feed line parameter monitoring module 71 detects thatthe degree of change in the transient voltage signal and transientcurrent signal as exceeding a set threshold, it places the infrared,sound, ultrasound, or magnetic field signal as a trigger signal totrigger the synchronization signal pickup module 72. The specific methodof notification that may be employed includes any of the following:

method one: the trigger signal is transmitted directly to the feedermonitoring device on the other two phase feeders for the signalsynchronization module 72;

method two: the first trigger signal is transmitted to the monitoringdevice in an adjacent phase, addressing the second signalsynchronization module 72, and then from the adjacent monitoring device,to the third phase feeder monitoring device, addressing the third signalsynchronization module 72;

method three: first, the trigger signal is transmitted to thecommunication terminal apparatus, and from there, relayed to the othercommunication terminal devices at the other two phases, and from there,to the other two monitoring devices, for the corresponding signalsynchronization modules 72.

Specifically, the said feeder monitoring device may further comprise:

a wireless communication module 74, for communicating with the other twophase feeder monitoring devices that correspond to a location, toestablish wireless communication by time-division multiplexing, and sendtogether including the parameters from the other two phases, to thesystem central unit via a wireless communication network, with theparameters include feeder current and voltage signals, feedertemperature, and the like.

Specifically, the said feeder monitoring device may further comprise:

a power requisition module 75, which has the ability to obtainelectrical power from a closed magnetic circuit by a latching mechanismso to supply power to the feeder monitoring device;

a power management module 76 for controlling the power requisitionmodule 75 to obtain electrical power to power the feeder monitoringdevice, and to obtain power from a battery as a backup power supply,such that when the power supply is normal, to obtain power from thefeeder, and when the feeder cannot supply normal power, to obtain powerfrom the battery, in order to ensure proper power supply to the feedermonitoring device.

The power level module 77 is for controlling power requisition module 75to obtain a leveled electrical power so to supply a constant power tothe feeder. The power level module can draw power from the feeder toensure that the power is available when a low but sufficient current isflowing. It can continue to draw power when a large but not yetsaturated current is flowing.

For ease of understanding, a specific implementation structure of thefeeder monitoring device will be described below.

Specifically, as shown in FIG. 8, the structure is pertaining to thefeeder monitor device 100 (i.e., feeder monitoring unit) in an openstate before being installed with a clamping mechanism in a schematicview. The feeder monitoring apparatus comprises housing 100, an upperarm 101, the main body 104, and a lower transparent housing 105. Theupper arm 101 has two halves: 102 and 103. 102 contains part of theelectronic current sensor. 103 contains part of the power acquisitionmodule (i.e., the power management module 75 and the power level controlmodule 76). The remaining part of the power acquisition module(including the power management module 75), the remaining part of theelectronic current sensor, wireless synchronization trigger means (i.e.synchronization processing module 721), the wireless communicationapparatus (i.e., wireless communication module 74), processing means(i.e., parameter monitoring feeder module 71), the fault indicatingdevice (i.e. fault indication module 73) are located within the mainbody 104. Fault indicator lights are emitted through the lowertransparent housing 105. Each module can be specifically sealed into themain body 104 by using sealant.

The main component of the feeder power acquisition module within themonitoring device 100 is a closed magnetic circuit having a magneticelement, such as a circular or other similar closed magnetic circuithaving a magnetic element. In order to for the magnetic circuit tosurround the distribution feeder 106, the magnetic element is cut intotwo parts, one is located in the upper arm 103, the other is locatedwithin the body 104. To draw power with a coil wound on the magneticelement, the power level control circuit is connected to the coil (i.e.,the power level control module 76). In order to allow drawing enoughpower in the distribution feeder apparatus 106 even on a very lowcurrent flow, the two parts of the magnetic element are closed withlocking to achieve a tightly closed magnetic circuit. The locking deviceconsists of a closed lock spring located in the upper arm shaft 116, adamper and a magnetic element located in the upper arm 103 as pressingspring. The closed lock spring keeps the arm and the body in closecontact. The damper can slow the locking arm during impact force. Themagnetic member in 103 of the upper arm and magnetic member of 104 ofthe man body would achieve snug contact by the pressing spring element.

A traditional means for drawing power works if the current that flowsthrough the distribution feeder 106 is low enough, and would be able toget the power supply device 100 operated. But when a very largedistribution current is flowing through the feeder 106, the magneticelement is saturated. The traditional means cannot get enoughelectricity. Therefore, be able to draw power from the distributionfeeder 106 if the current flowing through is large, the magnetic elementis made not to be saturated. But such modification would not work whenthe current flowing through the distribution feeder 106 is low, becausethere is not enough power to feed to the monitoring device 100'soperation. By using the power level control circuit of the presentinvention, magnetic element can draw enough power from the distributionfeeder 106 even when the current flowing through it is low to supply thepower to the feeder monitoring device 100. When the current flowingthrough the distribution feeder 106 is large, the power level controlcircuit can prevent the magnetic element from being saturated,therefore, still be able to supply enough power to the feeder monitoringdevice 100.

The power management unit within the feeder monitoring device 100 willconvert the power from the power acquisition unit to the device'soperating power. But when the distribution feeder power is off or whenthe power is not enough, the power management unit automaticallyswitches to the backup battery power. When the distribution feeder hasenough power again, the power management unit automatically switchesback again to draw power from the feeder.

Feeder monitoring device 100 can specifically incorporate an integratedvoltage and temperature sensor 117 for voltage and temperature signals.Sensor 117 is made of stainless steel, and is in close contact withdistribution feeder. The material has good electrical conductivity andthermal properties, so it can pick up the voltage signal on the feederand at the same time also can transfer heat to the thermal sensingelement.

Feeder monitoring device 100 picks up current signal by an electroniccurrent sensor. The electronic current sensor may be a currenttransformer CT, such as wound Rogowski coil, or a printed circuit boardtype Rogowski coil. The magnetic circuit electronic current sensor ispartially located in the upper arm 102, with the other portion locatedwithin the body 104.

Feeder monitoring device 100 is clamped onto the feeder by a holdingmeans. In the case of using an electronic Rogowski coil current sensor,the feeder would be in the center of Rogowski coil magnetic circuit. Thefeeder monitoring device 100 holding means is comprised of four latchingwire springs 107, 108, 109, 110, and four spring pins 111, 112, 113, 114for fixing the springs, as well as wire bracket 115.

Referring to FIG. 9, the schematic is showing the structure of thefeeder monitoring device 100 in accordance with one embodiment of thepresent invention being in a locked state using the holding means. Inthe locked state, the holding means' four springs 107, 108, 109, 110hold staggered and tightly onto the feeder 106, with feeder 106 beingfixed to the wire bracket 115.The power acquisition means' two portionsof the magnetic element would achieve snug contact with each other. Thesame can be said that the feeder 106 is now located in the center of theclosed magnetic circuit current sensors, with the integrated voltagetemperature sensor 117 and the distribution feeder 106 to achieve a snugfit.

According to another embodiment of the structure of a feeder monitoringdevice 100 of the present invention as shown in FIG. 10, the embodimentis comprising: an electrical coil 202 is wound on the electromagneticpower acquisition element 201, the feeder 212 is located in the centerof the magnetic circuit of the magnetic elements, with coil 202connected to the power level control device 203 (i.e., the power controlmodule 76), with the battery 204 and the power acquisition controlcircuit 203 connected to power management device 205 (i.e., the powermanagement module 75). Power supply means 205 takes the power outputfrom the power level control device 203 and the output of battery source204,and converts them into the needed power for the feeder monitoringdevice 100, as well as switches between the power level control unit 203and battery 204.

The processing device 208 (i.e. the feeder parameter monitoring module71) is for collecting from the integrated voltage and temperature sensor206 the voltage signal voltage signal, and for processing the signal inband-pass to derive the transient voltage signal. It extracts theamplitude, average, differential value, integral value of the transientvoltage signal and combinations thereof. When a change in the value ofone or more parameters exceeds the threshold, it triggers an alarm forthe suspected ground fault.

The processing means 208 also collects from the electronic currentsensor 207 the current signal, and processes the current signal inband-pass to derive the transient current signal. It extracts theamplitude, average, differential value, integrated value of thetransient current signal and combinations thereof. When a change in thevalue of one or more parameters exceeds the threshold, it triggers analarm for the suspected ground fault.

After the processing means 208 triggers a suspected ground fault alarm,it immediately triggers the synchronization device 210 via a wirelesstrigger to the other phase feeder monitoring devices 100.

The processing apparatus 208 is using the wireless communication device209 to externally establish time division multiplexing wirelesscommunication, and to synchronize time, to ensure any phase monitoringdevice 100 is in synced with the other phase feeder monitoring devicesmonitoring unit 100.

The feeder monitoring device 100 further comprises an indicator means211 (i.e., fault indication unit) for indicating a local ground fault.

According to another embodiment of the structure of a feeder monitoringdevice 100 of the present invention as shown in FIG. 11, the embodimentcomprises of a fault indication means. The fault indication means mayuse three wide-angle LEDs 301, 302, 303, spaced at 120 degrees apart.Upon receipt of a ground fault signal, it uses different intervals anddifferent flashing patterns to indicate different types of ground fault.

According to an embodiment of the low current single-phase ground faultdetection and location feeder monitoring system, as shown in FIG. 12,the embodiment specifically includes the said feeder monitoring device121, and fault location unit 122.

The fault location unit 122 is used to calculate the zero-sequencevoltage and the zero-sequence current based on the synchronouslycollected voltage and current signals at the plurality of locations onthe three-phase feeder lines. It also collects the steady-state andtransient signals based on the zero-sequence voltage and zero-sequencecurrent on the three-phase feeders at each location. Based on thesteady-state signals from the zero-sequence voltage and thezero-sequence current, it can decide which the fault line is. Similarly,based on the transient signals from the zero-sequence voltage and thezero-sequence current, it can decide where the specific fault on theline is. The fault locator unit specifically can be set up at the systemcentral unit. The system central unit and feeder monitoring devicescommunicate over remote communication means. Such communication is basedon one -to-many communication networking.

In particular, the fault location unit may comprise:

fault line module 1221 for extraction from the steady signals at variouslocations the zero sequence voltage and zero sequence current, thecharacteristic values of the steady state signals, the characteristicvalues include: amplitude, average, differential value and integralvalue, or one or more of those, as well as the zero sequence activepower and reactive power sequence, and calculation of the similarityfrom steady-state waveform zero-sequence voltage and zero-sequencecurrent signals at the various locations; determination of fault linebased on the differences in the steady state signal characteristicvalues and the waveform similarity; and

fault line positioning module 1222 for extracting at the respectivepositions at the fault line the zero-sequence voltage and zero-sequencecurrent transient signals fault location module, calculating thetransient signal characteristic values, with the characteristic valuesinclude: amplitude average, differential value and integral value of oneor more of the various positions' transient zero sequence active powerand reactive power zero-sequence, also calculating the waveformsimilarity of the transient zero sequence voltage and zero-sequencecurrent signals, and determining the ground fault point based ondifferences in the transient waveform similarity and the characteristicvalues at each position.

Further, the system may also include a fault indication display andindication processing unit 123, located at the system central unit, forafter determining the fault line position, showing it on the geographicinformation system (GIS) map, and sending a ground fault signal in awireless communication network to a fault indication unit at the groundfault point display to show the ground fault, so to not only show theground fault location at the system central unit, but also show at theground fault point that a local ground fault has occurred, in order tolet inspectors easily discover the location of a ground fault.

In order to facilitate communication between feeder monitoring devicethe system central unit, the system may also include a communicationterminal device 124. The corresponding communication terminal device 123may be used to achieve one of the following specific processes.

(1) Process for the feed line monitoring device communication: totransmit the feeder monitoring parameters (such as voltage and currentsignals, etc.) to the system central unit's fault location unit, witheach of the said terminal devices is for communicating with the one ormore sets of 3 of the three-phase feeder monitoring devices, based onthe time division multiplexing method to establish wirelesscommunication between a communication terminal device and the feederline monitoring devices, with the said communication terminalcommunicates with the said system central unit via a wirelesscommunication network.

(2) Process for communicates directly with any one of the three-phasefeeder devices, and transmit all three feeder monitoring devices'parameters up to the system's central fault locating unit. In order tocollect parameters from the other two feeder monitoring devices,wireless communication is established between the feeder monitoring andrest of the two feeder monitoring devices, with time divisionmultiplexing method used. The said communication terminal device and thesaid system central nit communicate via a wireless communicationnetwork.

With the said communication terminal equipment, the parameters from thefeeder monitoring devices can be easily transmitted to the systemcentral unit for the system central unit's fault location unit toperform ground fault location process.

The said system can also include feeder monitoring devices that havebuilt-in communication terminal functions, for communicating with theother two-phase feeders in the corresponding set, by time-divisionmultiplexing wireless communication, so to collect parameters from theother two feeder monitoring devices. The said feeder monitoring deviceswith built-in communication terminal functions communicate via awireless communication network with the said system central unit, andsend the three-phase feeder parameters to the system central unit'sfault locating unit

Based on the above said low current grounded single-phase ground faultdetection and location system, the present invention provides anembodiment on low current grounded single-phase ground fault detectionand location process, with the following specific steps:

Step one, install feeder monitoring devices (i.e. feeder monitoringdevices) on the three-phase distribution network feeders, with feedermonitoring device drawing power from the feeder controlled by powerlevel, with battery as a backup power supply.

Step two, the feeder monitoring devices pickup from each phase thevoltage signals through capacitive voltage sensors, and pick up fromeach phase the current signals through electronic current sensors.

Step three, each phase feeder monitoring device processes the collectedvoltage signals and band-pass current signals to extract the transientvoltage signals and current signals and calculates the amplitude, theaverage, the differential value, the integral value and theircombinations, when a change in the one or more values exceeds apredetermined threshold, triggers a suspected ground fault alarm.

Step four, after the fault line phase feeder monitoring device triggersthe suspected ground fault alarm, based on a wireless synchronizationtrigger (i.e. trigger signal), notify the other two phases to uploadvoltage and current waveform data.

Step five, after the other two-phase feeder monitoring devices receivethe trigger signal, using time-division multiplexing wirelesscommunication network timing and GPS timing, to accurately achieve timematching—meaning timing synchronization.

Step six, based on time division multiplexing in wireless communicationnetworks, collect feeder voltage and current waveform data for eachcorresponding position in the three-phase feeders, and calculate foreach position the zero-sequence voltage and zero-sequence current.

Step seven, extract at each position the zero-sequence voltage andzero-sequence current for the steady state signals, calculate thecharacteristic values, with the characteristic values include:amplitude, average, differential value, integral value and a combinationof those; also calculate the steady zero sequence active power,zero-sequence reactive power; calculate further the steady-statewaveform similarity of the zero-sequence voltage and zero-sequencecurrent signals; and based on differences in the steady-statezero-sequence voltage, zero-sequence current characteristic values andwaveform similarity for fault line and non-fault lines, performselection of the fault line and non-fault lines.

Step eight, extract at each position the zero-sequence voltage andzero-sequence current for the transient signals, calculate thecharacteristic values, with the characteristic values include:amplitude, average, differential value, integral value and a combinationof those; also calculate the steady zero sequence active power,zero-sequence reactive power; calculate further the transient statewaveform similarity of the zero-sequence voltage and zero-sequencecurrent signals; and based on the before fault and after fault locationsthe differences in the transient zero-sequence voltage, zero-sequencecurrent characteristic values and waveform similarity, analyze for eachpoint, especially with priority for the selected points at the suspectedfault line, to determine the ground fault point.

Step nine, at the determined ground fault point, combined with GIS(geographic information system, geographical information system) to showthe ground fault point on the map; at the same time issued a groundfault signal to the feeder monitoring unit's fault indication module toshow indication, so that the inspectors can locate the ground fault.

In the low current grounded single-phase ground fault detection andlocation system, the components of the system can include the feedermonitoring units, the communication terminals (i.e. the communicationterminal equipment) and the system central unit. Each communicationterminal can connect one or more sets of three-phase feeder monitoringunits. Each communication terminal and the three-phase feeder monitoringunits communicate using time division multiplex wireless communicationmethod, forming a wireless communication network. The communicationterminal and the system central unit communicate over wirelesscommunication network, such as GSM/GPRS, CDMA, and WIFI Ethernet, etc.

Alternatively, the said components of the system may include only thefeeder monitoring units and the system central unit. In such system, oneof the feeder monitoring units in the three-phase feeders (such as thefeeder monitoring unit at the B-phase feeder) functions simultaneouslyas a communication terminal. It communicates with the other two phasemonitoring units by time division multiplexing wireless communicationmethod, forming a wireless communication network. At the same time, thisfeeder monitoring unit and the system central unit communicate overwireless communication network, such as GSM/GPRS, CDMA, and WIFIEthernet, etc.

Alternatively, the said components of the system may include the feedermonitoring units, the communication terminals and the system centralunit. In such system, one of the feeder monitoring units in thethree-phase feeders (such as feeder monitoring unit at the B-phasefeeder) communicates with the other two phase monitoring units by timedivision multiplexing wireless communication method, forming a wirelesscommunication network. At the same time, this feeder monitoring unit andthe communication terminal communicate by time division multiplexingwireless communication method in another frequency, forming anotherwireless communication network, with each communication terminalcommunicating to multiple feeder monitoring units. The communicationterminal and the system central unit communicate over wirelesscommunication network, such as GSM/GPRS, CDMA, and WIFI Ethernet, etc.

In the system, if the system central unit and the communicationterminals are using GPS timing, for the feeder monitoring units tocommunicate to the communication terminals, it can use GPS timing.Normally when the feeder monitoring units use time division multiplexingwireless communication network to synchronize time, with the additionaluse of GPS timing together with the time division multiplexing wirelesscommunication network timing synchronization, one can achieve a precisesystem timing synchronization.

For ease of understanding, in conjunction with the following figures,the said system ground fault location process is described in detail.

As illustrated in FIG. 13A, when a low current grounded distributionfeeder with a single-phase ground fault occurs, during the ground faulttransient process, the voltage and current on both sides of the groundfault change dramatically within a very short period of time, andproduce abnormal transient voltage and current signals.

The system's distribution network feeder monitoring units use capacitivevoltage sensors, thus do not produce PT (voltage transformer)ferromagnetic resonance problems, therefore, they can reliably pick upthe transient voltage signals. The system feeder monitoring units useelectronic current sensors for current measurement, with high accuracyand good linearity, so that the measurement of low currents and largecurrents would have a high precision. In case of large current, becauseit is not saturated, transient characteristics are good, therefore, theunits can reliably pick up the transient current signal. Thus, in thesingle-phase ground fault transient process of a low current groundeddistribution feeder, because in the faulty phase the distribution feedermonitoring units use the capacitive voltage sensors and the electroniccurrent sensors, they can detect anomalies in the transient voltagesignals and the transient current signals.

As illustrated in FIG. 13B, in the distribution network faulty phase(i.e. C phase), after the feeder monitoring unit detects the suspectedground fault, it synchronously triggers (by sending a trigger signal totrigger) through wireless, a synchronized upload of the monitoredvoltage and current waveform data, from other phases' feeder monitoringunits. Referring to FIG. 13B, after a ground fault occurs at a C phaseposition, the adjacent two C phase feeder ground fault monitoring unitswill detect the suspected ground fault, and will send to at theirrespective corresponding positions of the other two phases (i.e., A andB phase) those feeder monitoring units, the trigger signal.

As illustrated in FIG. 13C, a three-phase distribution feeder monitoringunits can transmit the voltage and current waveform data via wirelesscommunication. Specifically, the voltage and current waveform data ofthe three-phase feeder monitoring unit scan be transmitted to thecommunication terminal, or by way of first converge the voltage andcurrent waveform data elements from two of the three-phase feedermonitoring units to the third phase feeder monitoring unit, follow byusing the communication terminal or that feeder monitoring unit toupload the voltage and current waveform data to the system centralunit's platform software, in order for the system central unit'splatform software to perform the corresponding ground fault pointlocating processing.

As illustrated in FIG. 13D, the system central unit's platform softwareperforms the fault detection and location, and issues the ground faultsignal to indicate the fault position, with the following specificprocedures:

(1) After obtaining from the three-phase feeder monitoring units atmultiple locations the voltage and current waveform data, calculate thezero-sequence voltage and zero-sequence current waveform;

(2) At the system central unit's platform software, from the calculatedzero-sequence voltage and zero-sequence current at each position,extract at each position the zero-sequence voltage and zero-sequencecurrent for the steady state signals, calculate the characteristicvalues, with the characteristic values include: amplitude, average,differential value, integral value and a combination thereof; alsocalculate the steady zero sequence active power, zero-sequence reactivepower; calculate further the steady-state waveform similarity of thezero-sequence voltage and zero-sequence current signals; and based ondifferences in the steady-state zero-sequence voltage, zero-sequencecurrent characteristic values and waveform similarity between fault lineand non-fault lines, perform selection of the fault line and non-faultlines;

(3) At the system central unit's platform software, from the calculatedzero-sequence voltage and zero-sequence current at each position,extract at each position the zero-sequence voltage and zero-sequencecurrent for the transient signals, calculate the characteristic values,with the characteristic values include: amplitude, average, differentialvalue, integral value and a combination of those; also calculate thesteady zero sequence active power, zero-sequence reactive power;calculate further the transient state waveform similarity of thezero-sequence voltage and zero-sequence current signals; and based onthe before fault and after fault locations the differences in thetransient zero-sequence voltage, zero-sequence current characteristicvalues and waveform similarity, analyze for each position, especiallywith priority for selected positions at the suspected fault line, todetermine the ground fault point;

(4) After the system central unit's platform software positioning theground fault point, the result can be set to have a manual secondaryreview (i.e., manual review by a manual review interface) to furtheraccurately determine the final ground fault position. The result isdisplayed on the GIS map for the ground fault point. At the same timethe system central unit's platform software system by time divisionmultiplexing wireless communication network, issues a ground faultsignal to the feeder monitoring unit to indicate the ground fault, so toconveniently allow manual line trace to locate the ground faultposition.

Through the above steps (1) through (4) of the process, it can easilyand accurately achieve power distribution feeder network ground faultpositioning with precision.

Before the system central unit conducts the ground fault positioningprocess, referring to FIG. 13B, the feeder distribution network faultmonitoring unit will detect the suspected ground fault. At that time, itcan be synchronized to trigger the other phase feeder monitoring unitsto synchronously upload wirelessly the monitored voltage and currentwaveform data.

The synchronization process wirelessly triggered by the correspondingfeeder monitoring unit specifically can use infrared, sound, ultrasonic,magnetic or electromagnetic field, with any one or combination thereof,sent a trigger signal to the other two phases. At the same time, thatfirst feeder monitoring unit can also receive wireless synchronizationtrigger signal. The wireless synchronization trigger may include thefollowing types:

(1) As shown in FIG. 14A, assume that the A-phase feeder monitoring unitdetects the suspected ground fault, the A-phase feeder monitoring unitvia a synchronous wireless triggering device, triggers synchronously theB-phase feeder monitoring unit and the C-phase feeder monitoring unit;

(2) As shown in FIG. 14A, assume that the A-phase feeder monitoring unitdetects the suspected ground fault, the A-phase feeder monitoring unitvia a synchronous wireless triggering device, triggers synchronously theB-phase feeder monitoring unit, and follows by the B-phase feedermonitoring unit triggers synchronously via wireless the C-phase feedermonitoring unit;

(3) As shown in FIG. 14C, assume that the A-phase feeder monitoring unitdetects the suspected ground fault, the A-phase feeder monitoring unitvia a synchronous wireless triggering device, triggers synchronously thecommunication terminal, follows by the communication terminal triggerssynchronously via wireless the B-phase feeder monitoring unit and theC-phase feeder monitoring unit.

The topology structure of the system according to an embodiment of thepresent invention can be referred to FIGS. 15A to 15D, wherein:

Structure I: As shown in FIG. 15A, the corresponding system topologycomprises: the feeder monitoring units, the communication terminal andthe system central unit. Each communication terminal is connected to aset of three-phase feeder monitoring units. The communication terminaland the three-phase feeder monitoring units communicate usingtime-division multiplexing wireless communication, forming a wirelesscommunication network. The communication terminal and the system centralunit communicate over GSM/GPRS, CDMA, and WIFI Ethernet. The systemcentral unit and the communication terminals are using GPS timing. Thecommunication terminal and the feeder monitoring units use time divisionmultiplexing wireless communication network to synchronize time, so tofinally achieve precisely synchronized system timing.

Structure II: As shown in FIG. 15B, the corresponding system topologycomprises: the feeder monitoring units, the communication terminal andthe system central unit. Each communication terminal is connected tomultiple sets of the three-phase feeder monitoring units, with FIG. 15Bshowing the communication terminal is connected to two sets of thethree-phase feeder monitoring units. The communication terminal and themultiple sets of three-phase feeder monitoring units communicate usingtime-division multiplexing wireless communication, forming a wirelesscommunication network. The communication terminal and the system centralunit communicate over GSM/GPRS, CDMA, and WIFI Ethernet. The systemcentral unit and the communication terminals are using GPS timing. Thecommunication terminal and the feeder monitoring units use time divisionmultiplexing wireless communication network to synchronize time, so tofinally achieve precisely synchronized system timing.

Structure III: As shown in FIG. 15C, the corresponding system topologycomprises: the feeder monitoring units, and the system central unit. Oneof the three-phase monitoring units such as B-phase unit would alsofunction as a communication terminal, and communicates to the other twophase feeder monitoring units using time-division multiplexing wirelesscommunication, forming a wireless communication network. This feedermonitoring unit and the system central unit communicate over GSM/GPRS,CDMA, and WIFI Ethernet. The system central unit and the communicationterminals are using GPS timing. This communication terminal/feedermonitoring unit and the other feeder monitoring units use time divisionmultiplexing wireless communication network to synchronize time, so tofinally achieve precisely synchronized system timing.

Structure IV: As shown in FIG. 15D, the corresponding system topologycomprises: the feeder monitoring units, the communication terminal andthe system central unit. One of the three-phase monitoring units such asB-phase unit communicates to the other two phase feeder monitoring unitsusing time-division multiplexing wireless communication, forming awireless communication network. At the same time, this feeder monitoringunit and the communication terminal communicate on another frequency,using time-division multiplexing wireless communication, forming anotherwireless communication network. Each communication terminal is connectedto multiple feeder monitoring units, with FIG. 15D showing thecommunication terminal is connected to two the feeder monitoring units.The communication terminal and the system central unit communicate overGSM/GPRS, CDMA, and WIFI Ethernet. The system central unit and thecommunication terminals are using GPS timing. The communication terminaland the feeder monitoring units use time division multiplexing wirelesscommunication network to synchronize time, so to finally achieveprecisely synchronized system timing.

With a system based on any of the above structure, the system centralunit's platform software can collect at each position in the three-phasedistribution network, from the feeder monitoring units, for the voltageand the current waveform data. It then can calculate for each positionthe zero-sequence voltage and zero-sequence current. The system centralunit's platform software, from the calculated zero-sequence voltage andzero-sequence current at each position, can extract at each position thezero-sequence voltage and zero-sequence current for the steady statesignals, calculate the characteristic values, with the characteristicvalues include: amplitude, average, differential value, integral valueand a combination thereof; also calculate the steady zero sequenceactive power, zero-sequence reactive power; calculate further thesteady-state waveform similarity of the zero-sequence voltage andzero-sequence current signals; and based on differences in thesteady-state zero-sequence voltage, zero-sequence current characteristicvalues and waveform similarity for fault line and non-fault lines,perform selection of the fault line and non-fault lines.

The system central unit's platform software, from the calculatedzero-sequence voltage and zero-sequence current at each position, canextract at each position the zero-sequence voltage and zero-sequencecurrent for the transient signals, calculate the characteristic values,with the characteristic values include: amplitude, average, differentialvalue, integral value and a combination thereof; also calculate thetransient zero sequence active power, zero-sequence reactive power;calculate further the transient state waveform similarity of thezero-sequence voltage and zero-sequence current signals; and based onthe before fault and after fault locations the differences in thetransient zero-sequence voltage, zero-sequence current characteristicvalues and waveform similarity, analyze for each position, especiallywith priority for the selected positions at the suspected fault line, todetermine the ground fault point.

After the system central unit's platform software positioning the groundfault point, the result can set to have a manual secondary review toconclusively determine the final ground fault position. The result isdisplayed on the GIS map for the ground fault point. At the same timethe system central unit's platform software system by time divisionmultiplexing wireless communication network, issues a ground faultsignal to the feeder monitoring unit to indicate the ground fault, so toconveniently allow manual line tracing to locate the ground faultposition.

The above are only some of the preferred embodiments of the presentinvention, but the scope of the present invention is not limited tothese. Any person skilled in the art and within the technical field ofthe present disclosure, if he can easily think of a change or areplacement, such change or replacement should fall within the scope ofthe invention. Accordingly, the protection scope of the presentinvention should be guided by the scope of the claims.

The invention claimed is:
 1. A low current single-phase ground faultdetection and location system comprising: a plurality of sets of threefeeder monitoring devices placed at a plurality of locations throughouta three-phase power distribution network, wherein each set includesthree feeder monitoring devices, and within each set, one feedermonitoring device is placed on each phase line of said three-phase powerdistribution network at a corresponding location of the plurality oflocations; and a ground fault location unit located remotely from saidfeeder monitoring devices configured to: calculate correspondingzero-sequence voltage and zero-sequence current for each of the sets offeeder monitoring devices at the plurality of locations based onsynchronously collected voltage and current signals by said feedermonitoring devices; calculate steady state signals and transient signalsfor each of the sets of feeder monitoring devices at the plurality oflocations based on the calculated corresponding zero-sequence voltageand zero-sequence current; determine a phase line for which a groundfault has occurred based on said calculated steady state signals; anddetermine a location of the ground fault on the determined phase linebased on said calculated transient signals and each said feedermonitoring device comprising: capacitive voltage sensors to detect saidvoltage signals, and electronic current sensors to detect said currentsignals; a feeder parameter monitoring module configured to process thedetected voltage and current signals, and send a notification tosynchronize signal collection to adjacent feeder parameter monitoringmodules in the same set based on said voltage and current signals; and atime synchronization module connected to the feeder parameter monitoringmodule, configured to receive a notification sent by an adjacentmonitoring device to synchronize collection of said voltage and currentsignals on each set of said feeder monitoring devices.
 2. The systemaccording to claim 1, wherein the ground fault location unit comprises:a ground fault line location module, configured to: calculate one ormore characteristic values of the steady-state signals, wherein thecharacteristic value includes any of amplitude, average, differentialvalue, or integral value; based on calculating steady zero sequenceactive power and zero-sequence reactive power, further calculatesteady-state waveform similarity of the steady-state signal; based ondifferences in the steady-state signal characteristic values and thewaveform similarity at each phase line, determine the selection of theground fault phase line; and a fault point positioning module,configured to: calculate at the plurality locations at the ground faultphase line indicated by the ground fault line location module, one ofmore characteristic values of the transient signal at each of theplurality of locations, wherein the characteristic value includes any ofamplitude, average, differential value, or integral value; based oncalculating at the plurality locations of the ground fault phase line,the transient zero sequence active power and reactive powerzero-sequence, further calculate the waveform similarity of thetransient signal; based on differences in the transient waveformsimilarity and the characteristic values at each location, determine theground fault location on the selected phase line.
 3. The systemaccording to claim 1, further comprises: a fault display and indicationprocessing unit configured to display the ground fault location on ageographic information system (GIS) map, and to send a ground faultsignal through a wireless communication network to a fault indicationunit near the ground fault location for display, wherein the faultindication unit is one of a plurality of fault indication units placednear the plurality of locations.
 4. The system according to claim 1,further comprises: a plurality of communication terminal devicesconfigured to communicate with the plurality of feeder monitoringdevices to relay the collected values of the current and voltage sensedto the ground fault locating unit wirelessly, wherein one communicationterminal device pairs with one set of feeder monitoring devices, suchthat: the communication terminal device is configured to use a timedivision multiplexing method to wirelessly communicate directly with allthree feeder monitoring device within the set; or the communicationterminal device is configured to wirelessly communicate directly withonly one of the three feeder devices acting as a proxy for the set,wherein the collection of values from the other two feeder monitoringdevices is accomplished by the proxy device; or one of the three feederdevices is configured to act as a proxy for the whole set would directlyincorporate the communication terminal device into itself, tocommunicate with the other two using time-division multiplexing wirelesscommunication.